SiC EPITAXIAL WAFER AND METHOD FOR MANUFACTURING SIC EPITAXIAL WAFER

ABSTRACT

A SiC epitaxial wafer of the present invention includes a SiC single crystal substrate, and a high concentration layer that is provided on the SiC single crystal substrate and has an average value of an n-type doping concentration of 1×10 18 /cm 3  or more and 1×10 19 /cm 3  or less, and in-plane uniformity of the doping concentration of 30% or less.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a SiC epitaxial wafer and a method formanufacturing the same.

Priority is claimed on Japanese Patent Application No. 2020-188693,filed Nov. 12, 2020, the content of which is incorporated herein byreference.

Description of Related Art

Silicon carbide (SiC) has a dielectric breakdown electric field that isone order of magnitude larger, a bandgap that is three times larger, anda thermal conductivity that is about three times higher than silicon(Si). For this reason, silicon carbide (SiC) is expected to be appliedto power devices, high frequency devices, high temperature operationdevices, and the like.

In order to promote the practical application of SiC devices, it isrequired to establish high-quality and low-cost SiC epitaxial wafers andepitaxial growth technology.

SiC devices are formed by using a SiC epitaxial wafer that includes aSiC substrate and an epitaxial layer formed on the SiC substrate. TheSiC substrate is obtained by processing a bulk single crystal of SiCgrown by a sublimation recrystallization method or the like. Theepitaxial layer is formed by a chemical vapor deposition (CVD) method orthe like and serves as a breakdown voltage maintaining region of adevice.

More specifically, the epitaxial layer is formed on the SiC substratewith a plane having an off angle in the <11-20> direction from the(0001) plane serving as a growth plane. The epitaxial layer is formed byperforming step-flow growth (lateral growth from an atomic step) on theSiC substrate to become 4H-SiC.

Basal plane dislocations (BPDs) are known as one type of device killerdefect that causes fatal defects in a SiC device on a SiC epitaxialwafer. For example, when a current is passed through a bipolar device inthe forward direction, partial dislocations of the BPDs inherited fromthe SiC substrate by the epitaxial layer move and expand (in otherwords, the BPDs extend and expand) due to recombination energy offlowing carriers, thereby forming high resistance stacking faults. Inaddition, when a high resistance portion is generated in the device,reliability of the device is lowered (forward deterioration). For thatreason, reduction of the BPDs inherited by the epitaxial layer has beenongoing.

Further, most of the BPDs in the SiC substrate can be converted intothreading edge dislocations (TEDs) in which expansion of defects doesnot occur when an epitaxial layer is formed (Patent Document 1).

However, it has become clear in recent years that, in a case in which alarge current is applied in the forward direction, the BPDs convertedinto the TEDs at an interface between the SiC substrate and theepitaxial layer also expand to stacking faults (SFs) in the epitaxiallayer. For that reason, in large-current power devices whose market isexpected to expand in the future, simply converting the basal planedislocations into the TEDs cannot sufficiently inhibit formation of theSFs, and there is always a concern of deterioration in reliability ofthe device.

Patent Document 2 discloses that, in addition to a normal epitaxiallayer, an epitaxial layer having a higher impurity concentration isformed in a SiC epitaxial wafer, and thus conversion efficiency fromBPDs to TEDs at an interface between a SiC single crystal substrate andthe epitaxial layer can be improved. By increasing the conversionefficiency of the BPDs, elongation and expansion of the BPDs can beinhibited. The elongation and expansion of the BPDs are a cause offorward deterioration of a device. For that reason, formation of theepitaxial layer having a high impurity concentration is considered to bea promising solution for inhibiting forward deterioration of a SiCdevice using a SiC epitaxial wafer.

Patent Document 3 discloses a manufacturing method for improvingin-plane uniformity in doping concentration of a low concentrationlayer. However, it does not mention in-plane uniformity in dopingconcentration of a high concentration layer.

PATENT DOCUMENTS

-   Patent Document 1: Japanese Unexamined Patent Application, First    Publication No. 2009-88223-   Patent Document 2: PCT International Publication No. WO2017/094764-   Patent Document 3: Japanese Patent No. 6386706

SUMMARY OF THE INVENTION

The present inventors have found previously unknown problems in which,when a SiC epitaxial wafer, which includes a SiC single crystalsubstrate and a normal epitaxial layer, and further includes an n-typeepitaxial layer having a high doping concentration (a high concentrationlayer) between them, is produced, in-plane uniformity (in the presentspecification, the in-plane uniformity of the doping concentrationindicates an “absolute value of (the maximum value of the dopingconcentration−the minimum value of the doping concentration)/an averagevalue of the doping concentration”) of an n-type doping concentration ofthe n-type epitaxial layer having a high doping concentration (highconcentration layer) deteriorates. In addition, as a result of diligentstudies, the present invention for solving such a problem has beenconceived.

The present invention has been made in view of the above circumstances,and an object thereof is to provide a SiC epitaxial wafer having highin-plane uniformity in n-type doping concentration of a highconcentration layer.

The present invention provides the following means for solving the aboveproblems.

(1) A SiC epitaxial wafer according to a first aspect of the presentinvention includes a SiC single crystal substrate, and a highconcentration layer that is provided on the SiC single crystal substrateand has an average value of a doping concentration of 1×10¹⁸/cm³ or moreand 1×10¹⁹/cm³ or less, and in-plane uniformity of the dopingconcentration of 30% or less.

The first aspect preferably includes the following features. One or moreof the following features may be combined with each other.

(2) In the SiC epitaxial wafer according to the above aspect, the highconcentration layer may be a buffer layer, and a drift layer having anaverage value of a doping concentration lower than an average value ofthe doping concentration of the buffer layer may be provided on thebuffer layer.

(3) In the SiC epitaxial wafer according to the above aspect, thein-plane uniformity of the high concentration layer may be 20% or less.

(4) In the SiC epitaxial wafer according to the above aspect, thein-plane uniformity of the high concentration layer may be 10% or less.

(5) The SiC epitaxial wafer according to the above aspect may have adiameter of the wafer of 150 mm or more.

(6) The in-plane uniformity may be obtained by using an expression of(an absolute value of (a maximum value of the doping concentration−aminimum value of the doping concentration)/an average value of thedoping concentration).

(7) The SiC epitaxial wafer may include a SiC epitaxial layer, and theSiC epitaxial layer may contain the high concentration layer.

(8) The SiC epitaxial layer may include the high concentration layer,and a drift layer that is provided on the high concentration layer andhas an average value of a doping concentration lower than that of thehigh concentration layer.

(9) The doping concentration may be an N doping concentration.

(10) As a manufacturing method according to a second aspect of thepresent invention, a method for manufacturing a SiC epitaxial wafer isprovided, which includes a step of preparing a chemical vapor depositiondevice, and a film formation step of forming a film by epitaxial growthon a SiC single crystal substrate in the chemical vapor depositiondevice, and the SiC epitaxial wafer that includes the SiC single crystalsubstrate, and a high concentration layer, that is provided on the SiCsingle crystal substrate and has an average value of an n-type dopingconcentration of 1×10¹⁸/cm³ or more and 1×10¹⁹/cm³ or less and in-planeuniformity of the doping concentration of 30% or less, is obtainedthrough the film formation step.

The second aspect preferably includes the following features. One ormore of the following features may be combined with each other.

(11) It is preferable that the film formation step includes a step offlowing a Si-based raw material gas, a C-based raw material gas, and adopant gas onto the SiC single crystal substrate to form an epitaxialfilm serving as the high concentration layer, and the epitaxial film isformed under a condition that an average value of an n-type dopingconcentration of the film is 1×10¹⁸/cm³ or more and 1×10¹⁹/cm³ or less,and a condition that a C/Si ratio, which is a molar ratio of C atoms inthe C-based raw material gas to Si atoms in the Si-based raw materialgas, is 1.1 or more and 1.7 or less.

(12) It is preferable that the chemical vapor deposition device has aplurality of Si-based gas supply pipes and a plurality of C-based gassupply pipes, wherein positions of the pipes which supply the gases tothe substrate are adjustable in an in-plane direction of the substrate.

(13) It is preferable to include, before the step of forming theepitaxial film, a step of determining a position of the C-based gassupply pipes of the chemical vapor deposition device, and a step ofdetermining a position of the Si-based gas supply pipes thereof.

(14) It is preferable that the step of determining the position of theC-based gas supply pipes has a first sub-step of supplying the dopantgas under a condition that a carrier concentration of an obtainedepitaxial film is less than 1×10¹⁷/cm³ and supplying the Si-based gasand the C-based gas under a condition of a C/Si ratio of 0.6 to 0.9 ontoa SiC single crystal substrate using the chemical vapor depositiondevice to obtain the epitaxial film, a second sub-step of confirming afilm thickness distribution of the epitaxial film obtained in the firstsub-step, a third sub-step of determining a position of the C-based gassupply pipes without moving them in a case in which the film thicknessdistribution is 10% or less and moving the position of the C-based gassupply pipes in a case in which the film thickness distribution is not10% or less, and a fourth sub-step of repeating the first to thirdsub-steps until the film thickness distribution becomes 10% or less, andthat the step of determining the position of the Si-based gas supplypipes has a first sub-step of supplying the dopant gas under a conditionthat a carrier concentration of an obtained epitaxial film is less than1×10¹⁷/cm³ and supplying the Si-based gas and the C-based gas under acondition of a C/Si ratio of 1.1 to 1.2 onto a SiC single crystalsubstrate using the chemical vapor deposition device, in which theposition of the C-based gas supply pipes has been determined in the stepof determining the position of the C-based gas supply pipes, to obtainthe epitaxial film, a second sub-step of confirming a film thicknessdistribution of the epitaxial film obtained in the first sub-step, athird sub-step of determining a position of the Si-based gas supplypipes without moving them in a case in which the film thicknessdistribution is 10% or less and moving the position of the Si-based gassupply pipes in a case in which the film thickness distribution is not10% or less, and a fourth sub-step of repeating the first to thirdsub-steps until the film thickness distribution becomes 10% or less.

According to the SiC epitaxial wafer of the present invention and themanufacturing method thereof, it is possible to provide a SiC epitaxialwafer having high in-plane uniformity in n-type doping concentration ofa high concentration layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing an example of a SiCepitaxial wafer according to one embodiment of the present invention.

FIG. 2 is a schematic cross-sectional view showing an example of a SiCepitaxial wafer according to another embodiment of the presentinvention.

FIG. 3 is a schematic cross-sectional view showing one example of aconfiguration for a device and a method with which an in-planedistribution of C-based gas (C-based raw material gas) supply andSi-based gas (Si-based raw material gas) supply can be independentlycontrolled.

FIG. 4 is a graph showing a relationship between an input nitrogen flowrate and an obtained doping concentration (average value) for threesamples shown in Table 1 when a C/Si ratio is 1.15.

FIG. 5 is a graph showing results of examining a relationship between anaverage value of the doping concentration and a growth rate.

FIG. 6 shows (a) a basic image diagram of nitrogen (N) doping, (b) animage diagram in a case in which the C/Si ratio is lower than that of(a), (c) an image diagram in a case in which the C/Si ratio is higherthan that of (a), and (d) a conceptual image diagram in a case in whicha doping flow rate is larger than that of (a).

FIG. 7 is a graph for describing how a substantially insufficient inputamount of C-based gas can be estimated from a calibration curve and anamount of decrease in growth rate.

FIG. 8 is a graph showing a relationship between an average value of thedoping concentration, in-plane uniformity of the doping concentration,and the C/Si ratio, which is shown in Table 1.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, examples of preferred embodiments of the present inventionwill be described with reference to the drawings. In addition, in eachof the following embodiments, the same or equivalent parts may bedesignated by the same reference numerals in the drawings. Also, in thedrawings used in the following description, featured portions may beenlarged for convenience in order to make the features easy tounderstand, and dimensional ratios and the like of each component arenot always the same as actual ones. Further, materials, dimensions, andthe like exemplified in the following description are examples, and thepresent invention is not limited thereto but can be carried out withappropriate changes as long as the effects of the present invention areachieved. A configuration shown in one embodiment can also be applied toother embodiments. For example, the present invention is not limited tothe following examples, and additions, omissions, substitutions, andchanges can be made to numbers, quantities, ratios, compositions, types,positions, materials, sizes, configurations, and the like withoutdeparting from the spirit of the present invention.

(SiC Epitaxial Wafer)

FIG. 1 is a schematic cross-sectional view showing an example of a SiCepitaxial wafer according to one embodiment of the present invention.FIG. 2 is a schematic cross-sectional view showing an example of a SiCepitaxial wafer according to another embodiment.

A SiC epitaxial wafer 100 shown in FIG. 1 includes a SiC single crystalsubstrate 10 and a SiC epitaxial layer 20 formed on a main surface 10 aof the SiC single crystal substrate 10.

The SiC epitaxial layer 20 included in the SiC epitaxial wafer 100 isconfigured of a high concentration layer 21 that has an average value ofa doping concentration of 1×10¹⁸/cm³ or more and 1×10¹⁹/cm³ or less, andin-plane uniformity of the doping concentration of 30% or less. Also,the doping concentration may be a concentration of a dopant element usedas a dopant, and may be a doping concentration of nitrogen (N), forexample.

In a SiC epitaxial wafer 200 shown in FIG. 2, a SiC epitaxial layer 20includes a high concentration layer 21, which is a buffer layer, and adrift layer 22 that is provided on the buffer layer and has an averagevalue of the doping concentration lower than an average value of thedoping concentration of the buffer layer.

<SiC Single Crystal Substrate>

For the SiC single crystal substrate 10, a substrate obtained by slicinga SiC ingot obtained by a sublimation method or the like can be used. Inthe present specification, the SiC epitaxial wafer is a wafer having anepitaxial layer after the epitaxial layer is formed, and the SiC singlecrystal substrate is a wafer having no epitaxial layer before theepitaxial layer is formed.

A size of the SiC single crystal substrate 10 is not limited, and forexample, a diameter thereof is preferably 100 mm or more, and morepreferably 150 mm or more.

For the SiC single crystal substrate 10, a substrate with a plane havingan offset angle in the <11-20> direction from (0001) serving as a growthplane can be used.

The SiC single crystal substrate 10 has basal plane dislocations alongthe (0001) plane (c plane). The number of the basal plane dislocationsexposed on the growth plane of the SiC single crystal substrate ispreferably small, but is not particularly limited.

In a case in which the SiC single crystal substrate 10 has a planehaving an offset angle in the <11-20> direction from (0001) as thegrowth plane, the basal plane dislocations are tilted with respect tothe growth plane.

The SiC single crystal substrate 10 may be formed under arbitraryconditions, and may be doped with nitrogen, for example. The dopingconcentration of the SiC single crystal substrate 10 is not particularlylimited, and a normal substrate used as a SiC substrate for powersemiconductors can be used.

<High Concentration Layer>

In the high concentration layer 21, an average value of an n-type dopingconcentration is 1×10¹⁸/cm³ or more and 1×10¹⁹/cm³ or less, and in-planeuniformity of the n-type doping concentration is 30% or less. That is, avalue obtained by the aforementioned expression is 0.30 or less (30% orless). The in-plane uniformity of the n-type doping concentration of thehigh concentration layer 21 is preferably 25% or less, more preferably20% or less, further preferably 15% or less, and particularly preferably10% or less. A lower value of the in-plane uniformity of the n-typedoping concentration is better for the quality of the SiC epitaxialwafer, but from the viewpoint of yield, a lower limit thereof can be setto 1% as an example of the lower limit. The lower limit of the in-planeuniformity may be 2%, 3%, or 5%. Dopants can be arbitrarily selected,and for example, nitrogen, ammonia, and the like can be used.

Regarding carriers in the epitaxial layer, in the basal dislocations ofthe SiC single crystal substrate at an interface between the epitaxiallayer and the SiC single crystal substrate, +carriers (holes) and−carriers (electrons) are recombined, and the basal plane dislocationsexpand to the epitaxial layer. The high concentration layer 21 havingcarriers at a high concentration inhibits carriers in the epitaxiallayer from reaching the SiC single crystal substrate.

In a case in which the high concentration layer 21 is a buffer layer,and the drift layer 22 is provided thereon, the high concentrationbuffer layer 21 and the drift layer 22 can be clearly distinguished dueto a difference in doping concentration (an average value of the dopingconcentration).

When the inventors produced a SiC epitaxial wafer having a highconcentration buffer layer and a drift layer in order on a SiC singlecrystal substrate by a general method, in-plane uniformity of a dopingconcentration of a high concentration buffer layer was 50% or more. Thatis, the in-plane uniformity was poor.

However, as a result of examination, it was found that a highconcentration layer having an in-plane uniformity of a dopingconcentration of 30% or less can be manufactured by using amanufacturing method, which will be described later. Also, for thein-plane uniformity of the doping concentration, a smaller valueindicates better uniformity, that is, the doping concentration isuniform and excellent in a plane.

When the in-plane uniformity of the doping concentration is poor, thereis a region in a surface of a wafer in which the concentration is lowerthan a target concentration. As a result, there arises a problem ofreducing an effect on carrier recombination, or a problem of causingdefects due to high concentration since there is a region in a surfaceof a wafer in which the concentration is higher than the targetconcentration. These problems can be prevented by setting the in-planeuniformity to a good state of 30% or less.

The high concentration layer 21 is n-type, and nitrogen is used as animpurity to be doped.

The average value of the n-type doping concentration of the highconcentration layer 21, that is, an average value of a nitrogenconcentration of the high concentration layer 21, is 1×10¹⁸/cm³ or moreand 1×10¹⁹/cm³ or less. For example, the average value of the n-typedoping concentration of the high concentration layer 21 may be1.2×10¹⁸/cm³ or more and 9×10¹⁸/cm³ or less, may be 1.5×10¹⁸/cm³ or moreand 8×10¹⁸/cm³ or less, may be 2×10¹⁸/cm³ or more and 7×10¹⁸/cm³ orless, may be 3×10¹⁸/cm³ or more and 6×10¹⁸/cm³ or less, or the like.

A film thickness of the high concentration layer 21 is not particularlylimited, and can be, for example, about 1 μm to 10 μm. For example, itmay be 2 μm to 8 μm, 3 μm to 6 μm, or the like. If it is too thin, aneffect of inhibiting carriers from reaching the SiC single crystalsubstrate is diminished, and if it is too thick, the cost becomes high.

A film thickness distribution of the high concentration layer 21 ispreferably 10% or less. The film thickness distribution may be a valueobtained from the expression represented by an absolute value of (themaximum value of a film thickness of the high concentration layer 21−theminimum value of the film thickness of the high concentration layer21)/an average value of the film thickness of high concentration layer21. That is, a value obtained by the above expression is preferably 0.10or less (10% or less).

This is because, when the film thickness distribution of the highconcentration layer 21 is 10% or less, at least one of an in-planedistribution of C-based gas supply on a substrate surface and anin-plane distribution of Si-based gas supply on the substrate surfacecan be as little as 10% or less, and thus a surface of the highconcentration layer 21 tends to be a mirror surface. The in-planedistributions of the gas supplies on the substrate surface can beobtained by measuring the film thickness of the epitaxial wafer formedunder a rate-determining condition, which will be described later.

<Drift Layer>

The high concentration layer 21 can be used as a buffer layer(intermediate layer), and the drift layer 22 can be provided on thebuffer layer (intermediate layer).

The drift layer 22 is a layer through which a drift current flows andfunctions as a device. The drift current is a current generated by aflow of carriers when a voltage is applied to a semiconductor. Thedoping concentration of the drift layer 22, that is, the nitrogenconcentration of the drift layer 22, is, for example, 1×10¹⁴/cm³ or moreand 1×10¹⁷/cm³ or less, and is usually about 1×10¹⁶/cm³.

<Method for Measuring Doping Concentration>

The n-type doping concentration of the high concentration layer can bemeasured by a mercury probe (Hg—CV) method or a secondary ion massspectrometry (SIMS) method.

In the Hg—CV method, N_(d)−N_(a) is measured as the n-type dopingconcentration. Here, N_(d) is a donor concentration and N_(a) is anacceptor concentration. If it is confirmed that N_(a) is sufficientlysmaller than N_(d), it can be considered that N_(d)−N_(a)≈Nd.

When the secondary ion mass spectrometry (SIMS) method is used,measurement is performed while scraping a high concentration layer in adepth direction of a SiC epitaxial wafer including the highconcentration layer, and thus the doping concentration of the highconcentration layer can be measured. The same applies to a SiC epitaxialwafer including a drift layer on a high concentration layer.

Measurement points may be any point as long as the distribution in thewafer surface can be reflected. However, the measurement points includeat least a wafer center, and a measurement point at a position 5 mm froma wafer edge, which is a measurement point farthest from the wafercenter. Points less than 5 mm from the edge are not included as themeasurement points.

As a specific procedure for measuring the n-type doping concentration ofthe high concentration layer, for example, in the case of a 6-inchwafer, the measurement may be performed in directions of a cross with acenter of the wafer set as an origin thereof at locations of the centerand a plurality of points on lines from the center toward an outerperiphery thereof. For example, the n-type doping concentration ismeasured at 21 points (example: 21 measurement points on two linesintersecting at a 90 degree angle at the center). The average value ofthe n-type doping concentration is calculated using the n-type dopingconcentration obtained at each point, and an absolute value of adifference between the maximum value and the minimum value of the n-typedoping concentration is divided by the calculated average value of then-type doping concentration. The in-plane uniformity can be obtained bythis calculation.

One of the directions of the cross can be parallel to an orientationflat.

<Other Layers>

The SiC epitaxial wafer of the present invention may include otherlayers as long as the effects of the present invention are achieved.

For example, another buffer layer (hereinafter referred to as a firstbuffer layer) of an n-type or p-type semiconductor having an impurityconcentration (the doping concentration) lower than that of the SiCsingle crystal substrate 10 may be provided between the SiC singlecrystal substrate 10 and the high concentration layer 21. The firstbuffer layer can be provided on the wafer in order to convert basalplane dislocations into threading edge dislocations. From this point ofview, the first buffer layer is a BPD conversion layer.

The impurity concentration of the first buffer layer is preferably lowerthan the impurity concentration of the SiC single crystal substrate 10and is preferably equal to or lower than the impurity concentration ofthe high concentration layer 21. As a value of the impurityconcentration of the first buffer layer, for example, the nitrogenconcentration is preferably 1×10¹⁷/cm³ or more. The value of theimpurity concentration of the first buffer layer is preferably1×10¹⁹/cm³ or less. For example, the value of the impurity concentrationof the first buffer layer may be 3×10¹⁷/cm³ or more and 8×10¹⁸/cm³ orless, may be 5×10¹⁷/cm³ or more and 4×10¹⁸/cm³ or less, may be7×10¹⁷/cm³ or more and 2×10¹⁸/cm³ or less, or the like.

The impurity concentration of the first buffer layer can be set to beintermediate between both impurity concentrations of the SiC singlecrystal substrate 10 and the high concentration layer 21 in order toalleviate a lattice mismatch between the SiC single crystal substrate 10and the high concentration layer 21.

(Method for Manufacturing SiC Epitaxial Wafer)

A method for manufacturing the SiC epitaxial wafer 100 or the SiCepitaxial wafer 200 according to the present embodiment is a method forperforming crystal growth of the epitaxial layer 20, for example, on theSiC single crystal substrate 10 whose main surface has an off angle of0.4° to 5° with respect to the (0001) plane.

First, the SiC single crystal substrate 10 is prepared. A method forproducing the SiC single crystal substrate 10 is not particularlylimited. For example, the substrate can be obtained by slicing a SiCingot obtained by a sublimation method or the like.

Next, the SiC epitaxial layer 20 (layers such as the high concentrationlayer 21 and the drift layer 22) is epitaxially grown on the preparedSiC single crystal substrate 10 to produce the SiC epitaxial wafer 100.The SiC epitaxial layer 20 can be formed by performing step-flow growth(lateral growth from an atomic step) on a growth plane 10 a of the SiCsingle crystal substrate 10 by a chemical vapor deposition (CVD) method.

A step of forming the SiC epitaxial layer 20 is performed by flowing araw material gas and a dopant gas on the SiC single crystal substrate,more specifically, on the SiC single crystal substrate held at a hightemperature.

The raw material gas is a gas serving as a raw material when the SiCepitaxial layer is formed. In general, for the raw material gas, aSi-based raw material gas containing Si in a molecule (Si-based gas) anda C-based raw material gas containing C in a molecule (C-based gas) canbe used in combination with each other.

As the Si-based raw material gas, a known gas can be used, and forexample, silane (SiH₄) can be mentioned. In addition, for the Si-basedraw material gas, a chlorine-based Si raw material-containing gas (achloride-based raw material) containing Cl having an etching function,such as dichlorosilane (SiH₂Cl₂), trichlorosilane (SiHCl₃), ortetrachlorosilane (SiCl₄), can also be used. For the C-based rawmaterial gas, for example, propane (C₃H₈), ethylene (C₂H₄), or the likecan be used.

The dopant gas is a gas containing an element serving as a donor oracceptor (carriers). In order to grow an n-type epitaxial layer,nitrogen, ammonia, or the like can be used for the dopant gas. In orderto grow a p-type epitaxial layer, trimethylaluminum (TMA),triethylaluminum (TEA), or the like can be used for the dopant gas.

In addition thereto, a gas or the like for transporting these gases intoa reactor may be used at the same time. For example, hydrogen, which isinert to SiC, can be used.

In a case in which the SiC epitaxial wafer 100 is manufactured, the stepof forming the SiC epitaxial layer 20 is a high concentration layer stepof forming the high concentration layer 21 on the SiC single crystalsubstrate 10. In a case in which the SiC epitaxial wafer 200 ismanufactured, the step of forming the SiC epitaxial layer 20 includesthe high concentration layer step of forming the high concentrationlayer 21 on the SiC single crystal substrate 10, and a drift layer stepof forming a drift layer on the high concentration layer 21.

<High Concentration Layer Step>

A growth temperature in the high concentration layer step can bearbitrarily selected, for example, to be 1400 to 1800° C., and is morepreferably 1500 to 1700° C. When the temperature is too low, polytypesother than 4H are likely to be generated. When the temperature is toohigh, surface roughness is likely to be generated.

In the present embodiment, the C/Si ratio is a molar ratio of C atoms inthe C-based raw material gas to Si atoms in the Si-based raw materialgas and means a C/Si ratio of the raw material gases. As for the C/Siratio of the raw material gases, as will be described later, the higherthe C/Si ratio of the raw material gases, the higher the in-planeuniformity of the doping concentration. On the other hand, when anattempt is made to increase the doping concentration, an amount ofdoping gas increases, and thus an effective C/Si ratio decreases. Thatis, for example, since N tries to enter a C site, a proportion of C thatcontributes to film formation decreases as compared with Si thatcontributes to the film formation. As described above, the C/Si ratio isa molar ratio of C atoms in the C-based raw material gas to Si atoms inthe Si-based raw material gas. Thus, the C/Si ratio of the raw materialgases to be supplied must be further increased to compensate for theabove-mentioned decrease. That is, for example, when an average value ofthe target doping concentration is 1×10¹⁸/cm³ to 5×10¹⁸/cm³, the C/Siratio is preferably 1.1 or more and 1.5 or less, and more preferably 1.2or more and 1.4 or less. Further, when the target concentration ishigher and 5×10¹⁸/cm³ to 1×10¹⁹/cm³, the C/Si ratio is also higher,preferably 1.3 or more and 1.7 or less, and more preferably 1.4 or moreand 1.6 or less.

That is, in the present embodiment, in order to form the highconcentration layer, an epitaxial film servings as the highconcentration layer is formed under the condition that the average valueof the n-type doping concentration of the film is 1×10¹⁸/cm³ or more and1×10¹⁹/cm³ or less, and the condition that the C/Si ratio, which is themolar ratio of the C atoms in the C-based raw material gas to the Siatoms in the Si-based raw material gas, is 1.1 or more and 1.7 or less.

When the high concentration layer is formed, it is necessary to dope alarge amount of impurities such as nitrogen in order to achieve a highconcentration. In this case, the C/Si ratio is lowered in general. Thatis, the proportion of C in the material gas used is lowered, and/or theproportion of Si is raised. This is because if an attempt is made todope at a high concentration while keeping the C/Si ratio as it is, itis necessary to introduce a large amount of doping gas. By lowering theC/Si ratio, the introduced amount of the doping gas is prevented frombecoming too large.

On the other hand, as a result of the inventors' diligent studies, ithas been found that lowering the C/Si ratio when the high concentrationlayer is formed causes a decrease in the in-plane uniformity of then-type doping concentration. In addition, it has been found that, whenthe high concentration layer is formed, the film formation is performedusing a higher C/Si ratio than usual, and thus it is possible to inhibita decrease in the in-plane uniformity of the n-type dopingconcentration. Further, it has been found that, in order to achievemirroring on the entire surface of the wafer, it is important that thein-plane distribution of the C-based gas supply and the Si-based gassupply on the substrate surface is good before the high concentrationlayer is formed.

<<Confirmation of In-Plane Distribution of C-Based Gas Supply andSi-Based Gas Supply on Substrate Surface>>

As will be described later, when the high concentration layer is formed,film formation (formation of the SiC epitaxial layer) is performed onthe SiC single crystal substrate using a predetermined C/Si ratio higherthan usual, and thus the high concentration layer whose entire surfaceis a mirror surface can be formed. However, a non-mirror surface regionmight be generated in a part thereof. This non-mirrored region oftenranged in size from 5% to 50% of the total area. By diligently studyingthe phenomenon in which the non-mirror surface region was generated, itwas found that a cause of the above generation was that the in-planedistribution of the C-based gas supply and the Si-based gas supply onthe substrate surface was not good. It was found that, in order toimprove this generation, it is effective to utilize a configuration of adevice, a manufacturing method, or the like that can independentlycontrol each in-plane distribution of the C-based gas and the Si-basedgas supply in gas supply to a film forming device such as a chemicalvapor deposition device. Also, in the present embodiment, the surface ofthe high concentration layer may be a mirror surface as a whole, or mayor may not have a non-mirror surface region, and a proportion of theregion is preferably 0 to 5%, more preferably 0 to 3%, furtherpreferably 0 to 2%, and particularly preferably 0 to 1%.

FIG. 3 shows a preferred example of a configuration in which thein-plane distribution can be controlled independently by supplying theC-based gas and the Si-based gas. In the present embodiment, a chemicalvapor deposition device having a plurality of Si-based gas supply pipesand a plurality of C-based gas supply pipes which can adjust gas supplypositions to the substrate in an in-plane direction of the substrate canbe preferably used. However, the present embodiment is not limited tothis example.

A vertical type film forming device 30 as shown in FIG. 3 has a gasintroduction unit. The gas introduction unit has gas supply units 32 a,32 b, and 32 c, which supply respective gases from an upper part to alower part in the device toward a placed substrate 10. The gas supplyunit may have each introduction port into which each gas is introducedinto the device, and each space to which each gas is supplied from eachsupply port. Specifically, the gas introduction unit includes a supplyunit 32 a for supplying only the C-based gas and a supply unit 32 b forsupplying only the Si-based gas. Each of the supply unit 32 a forsupplying only the C-based gas and the supply unit 32 b for supplyingonly the Si-based gas is configured to have a structure in which supplypositions toward the substrate 10 can be independently adjusted in ahorizontal direction (the in-plane direction of the substrate). Forexample, the positions of the gas supply pipes described below can beadjusted in the horizontal direction. In addition, for example, acarrier gas and/or dopant gas is supplied to the gas supply unit 32 c.Also, the carrier gas may also be supplied to the supply units 32 a and32 b. The gas supply unit 32 c may be a space in which film formation isperformed, or may not have a gas supply pipe. Further, a dopant gas maybe supplied from the C-based gas supply unit, from the Si-based gassupply unit, and/or from the carrier gas supply unit. In the exampleshown in FIG. 3, the C-based gas supply unit 32 a includes gas supplypipes 32 aa, 32 ab, and 32 ac, which are disposed at predeterminedarbitrary intervals and extend in a direction perpendicular to thein-plane direction. The Si-based gas supply unit 32 b includes gassupply pipes 32 ba and 32 bb, which are disposed at predeterminedarbitrary intervals and extend in the direction perpendicular to thein-plane direction. These gas supply pipes can be connected to each ofthe above spaces. The number of gas supply pipes for supplying theC-based gas and the number of gas supply pipes for supplying theSi-based gas may be the same or different. The number of gas supplypipes for each gas can be arbitrarily selected and may be, for example,2 to 40, 5 to 20, 8 to 15, or the like, but is not limited to theseexamples. Positions of end portions (gas discharge end portions) ofthese gas supply pipes may be at the same height when seen in a sideview.

Here, the in-plane distribution of the C-based gas supply on thesubstrate surface can be measured by measuring a film thicknessdistribution (a growth rate distribution) of the epitaxial wafer formedunder a C supply rate-determining condition, as will be described later.In the present specification, the in-plane distribution of the gassupply on the substrate surface indicates an “absolute value of (themaximum value of a film thickness of the epitaxial wafer−the minimumvalue of film thickness of epitaxial wafer)/average value of filmthickness of epitaxial wafer.” Further, the thickness of the SiCepitaxial wafer can be measured by a known method, for example, Fouriertransform infrared spectroscopy (FT-IR). The C supply rate-determiningcondition is a state in which supply of the C-based gas is insufficientas compared with the Si-based gas. The C supply rate-determiningcondition preferably is within the range of a C/Si ratio of 0.6 to 0.9.For example, it is conceivable to carry out the above film formationwith a C/Si ratio of 0.8. However, the film is formed under thecondition that a carrier concentration of the epitaxial wafer is, thatis, a doping concentration of the epitaxial film of the epitaxial waferis less than 1×10¹⁷/cm³ so that an input amount of an N-based gas suchas nitrogen does not change the effective C/Si ratio. It is alsopreferable to confirm the film thickness distribution of the obtainedepitaxial film and repeat change of the supply position of the C-basedgas and confirmation of the in-plane distribution of the obtained filmuntil a desired in-plane distribution (film thickness distribution) isachieved.

On the other hand, the in-plane distribution of the Si-based gas supplyon the substrate surface can be measured by measuring the film thicknessdistribution (growth rate distribution) of the epitaxial wafer formedunder a Si supply rate-determining condition. The Si supplyrate-determining condition is a state in which supply of the Si-basedgas is insufficient as compared with the C-based gas. The Si supplyrate-determining condition is preferably within the range of a C/Siratio of 1.1 to 1.2. For example, it is conceivable to carry out theabove film formation with a C/Si ratio of 1.1. However, it is preferableto form the film under the condition that the carrier concentration ofthe epitaxial wafer is less than 1×10¹⁷/cm³ so that an input amount ofthe N-based gas does not change the effective C/Si ratio. It is alsopreferable to confirm the film thickness distribution of the obtainedepitaxial film and repeat change of the supply position of the Si-basedgas and confirmation of the in-plane distribution of the obtained filmuntil a desired in-plane distribution (film thickness distribution) isachieved.

In this way, using the above configuration, for example, by the methoddescribed later, the in-plane distribution of the C-based gas supply(the film thickness distribution of the epitaxial wafer formed under theC supply rate-determining condition) and the in-plane distribution ofthe Si-based gas supply (the film thickness of the epitaxial waferformed under the Si supply rate-determining condition) are measured, andin a case in which the in-plane distribution of the C-based gas supplyis not 10% or less and the in-plane distribution of the Si-based gassupply is not 10% or less, the gas supply positions of the C-based gassupply unit and the Si-based gas supply unit, that is, the positions ofthe gas supply pipes are adjusted. For example, in a case in whichsupply of the C-based gas in the vertical type film forming device issmaller in the center and larger in the outer periphery, for example, ina case in which the film thickness of the film formed under the C supplyrate-determining condition is thinner at the center, the position of thesupply pipe of the C-based gas supply unit of the gas introduction unitis moved to the center side.

In this way, the present embodiment may include, before the step offorming the high concentration layer, a step of determining thepositions at which each gas is supplied in accordance with the filmthickness distribution of the film to be formed. Specifically, beforethe step of forming the high concentration layer, with respect to thedevice used for the formation, the present embodiment may include a stepof determining the position of the C-based gas supply pipe and a step ofdetermining the position of the Si-based gas supply pipe. These stepsare preferably performed under the C or Si rate-determining condition,which will be described later. Also, either step may be performed first.For example, it is preferable that a position of one supply pipe befirst determined, and then in a state in which the position ismaintained, the step of determining a position of the other supply pipebe performed.

It was confirmed that, in a case in which the in-plane distribution onthe substrate surface of the C-based gas supply, that is, the filmthickness distribution, is 10% or less, and the in-plane distribution ofthe Si-based gas on the substrate surface is 10% or less, the non-mirrorsurface region was not generated in the high concentration layer.

(Measurement of In-Plane Distribution in Growth Rate Under CRate-Determining and Si Rate-Determining)

Prior to preparing samples described below, supply positions of theC-based gas supply unit and the Si-based gas supply unit of the filmforming device were adjusted, and in-plane distributions in growth rateunder C rate-determining and Si rate-determining was measured. That is,a film thickness distribution of a formed film was measured. For themeasurement, a chemical vapor deposition (CVD) device as outlined inFIG. 3 was used.

(i) C Rate-Determining

A film was formed by adjusting an amount of nitrogen such that theobtained doping concentration was 8×10¹⁵/cm³ at a C/Si ratio of 0.8.Then, by adjusting the position of only the supply pipe of the C-basedgas supply unit, the in-plane distribution of the C-based gas supply onthe substrate surface was set to 5.6%. The position of the supply pipeof the Si-based gas supply unit was not adjusted.

(ii) Si Rate-Determining

A film was formed by adjusting an amount of nitrogen such that theobtained doping concentration was 1.3×10¹⁶/cm³ at a C/Si ratio of 1.1.Then, by adjusting the position of only the supply pipe of the Si-basedgas supply unit, the in-plane distribution of the Si-based gas supply onthe substrate surface was set to 3.4%. Further, the supply pipe of theC-based gas supply unit was left fixed at the position set under Crate-determining.

The in-plane distribution of the C-based gas supply and the in-planedistribution of the Si-based gas supply were measured at 21 points inthe wafer in the same manner as the in-plane uniformity of the n-typedoping concentration described below.

(Sample Preparation and Measurement of In-Plane Uniformity of n-TypeDoping Concentration of Sample)

Using the film forming device in which adjustment of the positions hasbeen completed, a 4H-SiC single crystal substrate having a diameter of150 mm and a main surface with an off angle of 4°, and nitrogen servingas an n-type dopant, samples in which a high concentration epitaxiallayer was formed on an Si surface of the substrate was produced whilevarying manufacturing conditions. Table 1 shows results of examining thein-plane uniformity of the n-type doping concentration of the highconcentration epitaxial layer of the manufactured samples. As forspecific manufacturing conditions and measurements, a plurality ofsamples (wafers) were manufactured by setting the C/Si ratio of the rawmaterial gases to any of 1.05, 1.15, and 1.35, and changing conditionsregarding a doping gas to be introduced, and 21 points in the surface ofthe wafer were measured. The in-plane uniformity is the in-planeuniformity of the n-type doping concentration of the high concentrationepitaxial layer of the wafer obtained by introducing the doping gasaiming at a predetermined value as an average value of the dopingconcentration. All samples were produced under the same conditionsexcept that the C/Si ratio and a flow rate of the doping gas (nitrogen)were changed. While the unit is set to mm, the center of the wafer is(0,0), and the orientation flat is in the Y direction, the measurementpoints are (X, Y)=(0, 70), (0, 60), (0, 45), (0, 30), (0, 15), (0, 0),(0, −15), (0, −30), (0, −45), (0, −60), (0, −67), (−70, 0), (−60, 0),(−45, 0), (−30, 0), (−15, 0), (15, 0), (30, 0), (45, 0), (60, 0), and(70, 0). The numerical values in parentheses are distances (mm) from thecenter.

Further, a higher doping concentration (average value) can be achievedby increasing the flow rate of supplied nitrogen. FIG. 4 shows arelationship between a relative value of the flow rate of suppliednitrogen and an obtained doping concentration (average value) in a casein which the flow rate of nitrogen varied for three samples when theC/Si ratio shown in Table 1 was set to 1.15. In FIG. 4, the horizontalaxis is a relative value of the flow rate of supplied nitrogen in a casein which the flow rate of supplied nitrogen is set to 1 when theobtained doping concentration (average value) is 1.03×10¹⁸/cm³. Thevertical axis is the obtained doping concentration (average value). Itcan be seen that the doping concentration increases as the flow rate ofnitrogen increases. As the film forming device, the vertical type deviceas shown in FIG. 3 was used.

TABLE 1 Average value of doping In-plane C/Si concentration (cm⁻³)uniformity 1.05 2.01E+18 31.6% 1.05 4.24E+18 43.9% 1.05 1.10E+19 115.8%1.15 1.03E+18 12.2% 1.15 1.91E+18 14.0% 1.15 2.59E+18 17.8% 1.352.20E+18 6.1%

As shown in Table 1, in the case of using the condition that a normallyused C/Si ratio is 1.05, even when the average value of the n-typedoping concentration is about 2×10¹⁸ cm⁻³, the in-plane uniformity ofthe n-type doping concentration exceeds 30%. Thus, it can be seen that,when the average value of the n-type doping concentration is increased,the value of the in-plane uniformity of the n-type doping concentrationincreases, and the in-plane uniformity further deteriorates.

On the other hand, in the case of using 1.15, which is higher than thenormally used C/Si ratio, even when the average value of the n-typedoping concentration is about 1×10¹⁸ cm⁻³, the in-plane uniformity valuedecreases, that is, the in-plane uniformity becomes 12.2%, and thusexcellent and high in-plane uniformity of the n-type dopingconcentration is obtained. When the average value of the n-type dopingconcentration is raised to about 2.6×10¹⁸ cm⁻³, the value of thein-plane uniformity slightly increases, that is, the in-plane uniformityof the n-type doping concentration slightly decreases, but the value ofthe in-plane uniformity is 17.8%, which is still good.

Further, in the case of using 1.35, which is a C/Si ratio much higherthan the normally used C/Si ratio, the value of the in-plane uniformityvalue decreases at the average value of the n-type doping concentrationof 2.2×10¹⁸ cm⁻³, that is, extremely excellent and high in-planeuniformity of the n-type doping concentration, i.e., 6.1%, was obtained.

From the above results, it has been found that, by using a C/Si ratiohigher than the normally used C/Si ratio of 1.05, the in-planeuniformity of the n-type doping concentration is improved, and thein-plane uniformity of the n-type doping concentration is furtherimproved as the C/Si ratio becomes higher. Also, it has been found thatincreasing the average value of the n-type doping concentrationdeteriorates the in-plane uniformity of the n-type doping concentrationregardless of which C/Si ratio is used.

FIG. 5 shows results of examining a relationship between the averagevalue of the n-type doping concentration and the growth rate for samplesof the SiC epitaxial wafer having a diameter of 150 mm by performingmeasurements under the same conditions except that the doping conditionwas changed. Also, as the film forming device, the vertical type deviceas shown in FIG. 3 was used.

In FIG. 5, the horizontal axis is the average value of the n-type dopingconcentration. The vertical axis is a standard value of the growth ratewhen the growth rate is standardized by setting the growth rate of theepitaxial film to 1 in a case in which epitaxial growth is performed sothat the average value of the n-type doping concentration is 1×10¹⁶cm⁻³.

As shown in FIG. 5, it can be seen that the growth rate tends todecrease for the high concentration layer having a higher average valueof the n-type doping concentration. This tendency is presumed to be dueto the fact that nitrogen (N) inhibits growth of SiC (site-competitioneffect). This point will be described below.

In FIG. 6, (a) to (d) are image diagrams of nitrogen (N) doping. Theyare figures showing a state in which nitrogen is about to enter a carbonlattice site of SiC. In FIG. 6, (b) shows a case in which the C/Si ratiois lower than that of (a), (c) shows a case in which the C/Si ratio ishigher than that of (a), and (d) shows a case in which the flow rate ofthe doping gas is larger than that of (a) (when a high average value ofthe doping concentration is aimed at). Here, the state at the time ofdoping shown in (a) to (c) of FIG. 6 has been conventionally known as asite-competition effect, the state at the time of doping shown in (d) ofFIG. 6 is a finding newly discovered by the present inventors. Further,in FIG. 6, the number x of C, the number y of Si, and the number z of Nin a gas phase shown for convenience are set to x=5, y=5, z=1 in (a),x=3, y=5, z=1 in (b), x=8, y=5, z=1 in (c), and x=5, y=5, z=5 in (d).

As shown in (a) of FIG. 6, Si is incorporated into a Si site, C isincorporated into a C site, and N is incorporated into a C site.

On the other hand, in the case of a low C/Si ratio, as shown in (b) ofFIG. 6, a ratio of N to C increases, and thus the probability of Nentering the C site increases.

Further, in the case of a high C/Si ratio, as shown in (c) of FIG. 6,the ratio of N to C decreases, and the probability of N entering the Csite decreases.

In addition, in a case in which N has a concentration as high as that ofthe raw material gases, the ratio of N to C increases as shown in (d) ofFIG. 6.

For that reason, C is less likely to be incorporated into the C site,and as a result, the effective C/Si ratio is lowered and the growth rateis lowered.

Here, the inventors have examined the relationship between an inputamount of the C-based gas and the growth rate by using the CVD devicethat has produced the samples for the measurement of the in-planeuniformity of the n-type doping concentration described above. As aresult, a calibration curve shown in FIG. 7 was obtained, which shows apositive correlation that the growth rate increases as the input amountof the C-based gas increases.

From this calibration curve and the amount of decrease in growth ratethat decreased by increasing the n-type doping concentration, it ispossible to estimate a substantially insufficient input amount of theC-based gas. This will be described with reference to the conceptualdiagram of FIG. 6 and FIG. 7.

In FIG. 7, the horizontal axis represents the input amount of theC-based gas, and the vertical axis represents the growth rate of the SiCepitaxial layer. Also, the input amount of the C-based gas correspondsto a C/Si ratio when an input amount of the Si-based gas input is fixed.

It is assumed that a SiC epitaxial wafer having a higher average valueof the n-type doping concentration than an average value of the n-typedoping concentration of the SiC epitaxial wafer obtained under thecondition of P1 on the calibration curve (the input amount of theC-based gas is C1, and the growth rate is R1) is produced. Referring tothe calibration curve of FIG. 7, if the flow rate of the doping gas isincreased and the growth rate is R3 instead of R1 when epitaxial growthis performed with the input amount of the C-based gas of C1, it isconsidered that an effective input amount of the C-based gas, that is,an amount of C-based gas that contributed to the growth of the film, wasC3. In this case, it is considered that since the ratio of N atoms to Catoms increased, it became difficult for C atoms to be incorporated intothe C site, and as a result, the effective input amount of the C-basedgas became C3. By adding an amount of C-based gas that compensate forthis shortage, epitaxial growth can be performed at the growth rate R1.

The in-plane uniformity of the doping concentration will be describedbelow with reference to Table 1 and FIG. 8.

As shown in Table 1, as a result of increasing the C/Si ratio tocompensate for the shortage of the C-based gas, the in-plane uniformityof the n-type doping concentration was significantly improved, and atthe time of the C/Si ratio=1.35, the in-plane uniformity was 6.1%.

When a SiC epitaxial layer having a normal concentration (for example,1×10¹⁶/cm³ or less) was formed instead of one having a high dopingconcentration under the condition of the C/Si ratio=1.35, it became anon-mirror surface. However, in the SiC epitaxial layer having a highdoping concentration formed under the condition of the C/Si ratio=1.35shown in Table 1, the entire surface was a mirror surface. This isconsidered to support that the nitrogen (N) atoms inhibit the growth ofSiC due to the above-mentioned site compensation effect.

When manufactured under the same C/Si ratio condition, the SiC epitaxiallayer having a high doping concentration is considered to be moreaffected by the site compensation effect than the SiC epitaxial layerhaving a low doping concentration. In addition, it is considered thatuniform growth of SiC on the growth plane is likely to be hindered, andas a result, the in-plane uniformity of the doping concentration is alsoimpaired. Accordingly, it is considered that, in the SiC epitaxial layerhaving a high doping concentration, the influence of the sitecompensation effect is reduced by increasing the C/Si ratio, and as aresult, it is possible to inhibit hindrance to uniform growth of SiC onthe growth plane, that is, to carry out uniform growth of SiC and toinhibit deterioration of the in-plane uniformity of the dopingconcentration.

In this way, from the correlation between the input amount of theC-based gas and the growth rate (for example, the calibration curve)acquired in advance, the amount of dopant that hinders the epitaxialgrowth of SiC is estimated, that is, the insufficient amount of C ispredicted, and the raw material gases are provided at the C/Si ratiothat can compensates for that amount, and thus the SiC epitaxial waferhaving high in-plane uniformity can be manufactured.

Also, in all of the samples obtained in Table 1, the entire surface ofthe high concentration layer was a mirror surface. That is, it has beenconfirmed that, in a case in which the high concentration layer wasmanufactured under the condition that the in-plane distribution of theC-based gas supply on the substrate surface is 10% or less and thein-plane distribution of the Si-based gas on the substrate surface is10% or less, a non-mirror surface region was not generated in the highconcentration layer. Thus, it has been found that, by setting the C/Siratio to a preferable range in addition to the conditions under whichthe mirror surface is manufactured, the entire surface of the wafer canbe a mirror surface, and the in-plane uniformity of the n-type dopingconcentration can be improved.

FIG. 8 is a graph showing, on the basis of the above experimentalresults and considerations, from the results shown in Table 1, arelationship between the average value of the n-type dopingconcentration, the in-plane uniformity of the n-type dopingconcentration, and the C/Si ratio for samples of the SiC epitaxial waferhaving a diameter of 150 mm.

By estimating growth conditions of the C/Si ratio and the doping gasflow rate on the basis of the relationship shown in FIG. 8, for example,by estimating them in advance and making a prediction, the SiC epitaxialwafer including the high concentration layer having the average value ofthe n-type doping concentration of 1×10¹⁸/cm³ or more and 1×10¹⁹/cm³ orless, and the in-plane uniformity of the n-type doping concentration of30% or less can be manufactured. For example, it can be seen from FIG. 8that it is preferable to consider increasing the C/Si ratio because theuniformity of the doping concentration deteriorates as the film having ahigher average doping concentration is desired.

Also, in FIG. 8, in the line shown in the graph in the case of the C/Siratio of 1.35, the data on the line under the above conditions is onepoint, but it is considered to be a line obtained on the basis of amechanism based on the site compensation effect described above. Thiswill be described.

In FIG. 8, the graph in the case of the C/Si ratio of 1.05 and the graphin the case of the C/Si ratio of 1.15 are obtained from the data ofthree points. The fact that these graphs have positive slopes, and theslope of the graph in the case of the C/Si ratio of 1.15 is lower thanthe slope of the graph in the case of the C/Si ratio of 1.05 shows thatthe mechanism based on the site compensation effect fits well. It meansthat the higher the C/Si ratio, the better the in-plane uniformity ofthe doping concentration. That is, the higher the C/Si ratio, the lowerthe value of the in-plane uniformity of the doping concentration.Further, the fact that, in the SiC epitaxial layer in the vicinity of2×10¹⁸/cm³, which is higher than the normal doping concentration, thein-plane uniformity of the doping concentration is more and moreimproved in the order of the C/Si ratio of 1.35, the C/Si ratio of 1.15,and the C/Si ratio of 1.05 also shows that the mechanism based on thesite compensation effect fits well.

In view of the above, it can be inferred that, in the case of the C/Siratio of 1.35, even within the doping concentration range of 1×10¹⁸/cm³or more and 1×10¹⁹/cm³ or less, the mechanism based on the sitecompensation effect works well.

In a case in which the mechanism based on the site compensation effectfits well, it is inferred that the graph in the case of the C/Si ratioof 1.35 has a positive slope, and the slope is lower than the slope ofthe graph in the case of the C/Si ratio of 1.15.

As described above, the graph in the case of the C/Si ratio of 1.35 hasbeen obtained on the basis of the mechanism based on the sitecompensation effect.

The present invention provides a SiC epitaxial wafer having highin-plane uniformity in doping concentration of a high concentrationlayer and a method for producing the same.

EXPLANATION OF REFERENCES

-   -   10 SiC single crystal substrate    -   10 a Main surface of SiC single crystal substrate    -   20 SiC epitaxial layer    -   21 High concentration layer    -   22 Drift layer    -   30 Film forming device    -   32 a Gas supply unit for supplying C-based gas    -   32 b Gas supply unit for supplying Si-based gas    -   32 c Gas supply unit for supplying carrier gas    -   32 aa, 32 ab, 32 ac C-based gas supply pipe    -   32 ba, 32 bb Si-based gas supply pipe    -   100 SiC epitaxial wafer    -   P1 Point on calibration curve    -   R1, R3 Growth rate    -   C1, C2, C3 C Input amount of C-based gas

1. A SiC epitaxial wafer comprising: a SiC single crystal substrate; anda high concentration layer that is provided on the SiC single crystalsubstrate and has an average value of an n-type doping concentration of1×10¹⁸/cm³ or more and 1×10¹⁹/cm³ or less, and in-plane uniformity ofthe doping concentration of 30% or less.
 2. The SiC epitaxial waferaccording claim 1, wherein the high concentration layer is a bufferlayer, and a drift layer having an average value of a dopingconcentration lower than an average value of the doping concentration ofthe buffer layer is provided on the buffer layer.
 3. The SiC epitaxialwafer according to claim 1, wherein the in-plane uniformity of the highconcentration layer is 20% or less.
 4. The SiC epitaxial wafer accordingto claim 1, wherein the in-plane uniformity of the high concentrationlayer is 10% or less.
 5. The SiC epitaxial wafer according to claim 1,wherein a diameter of the wafer is 150 mm or more.
 6. The SiC epitaxialwafer according to claim 1, wherein the in-plane uniformity is obtainedby using an expression of (an absolute value of (a maximum value of thedoping concentration−a minimum value of the doping concentration)/anaverage value of the doping concentration).
 7. The SiC epitaxial waferaccording to claim 1, wherein the SiC epitaxial wafer includes a SiCepitaxial layer, and the SiC epitaxial layer includes the highconcentration layer.
 8. The SiC epitaxial wafer according to claim 7,wherein the SiC epitaxial layer includes the high concentration layer,and a drift layer that is provided on the high concentration layer andhas an average value of a doping concentration lower than that of thehigh concentration layer.
 9. The SiC epitaxial wafer according to claim1, wherein the doping concentration is an N doping concentration.
 10. Amethod for manufacturing a SiC epitaxial wafer, comprising: a step ofpreparing a chemical vapor deposition device; and a film formation stepof forming a film by epitaxial growth on a SiC single crystal substratein the chemical vapor deposition device, wherein the SiC epitaxial waferthat includes the SiC single crystal substrate, and a high concentrationlayer, that is provided on the SiC single crystal substrate and has anaverage value of an n-type doping concentration of 1×10¹⁸/cm³ or moreand 1×10¹⁹/cm³ or less and in-plane uniformity of the dopingconcentration of 30% or less, is obtained through the film formationstep.
 11. The method for manufacturing a SiC epitaxial wafer accordingto claim 10, wherein the film formation step includes a step of flowinga Si-based raw material gas, a C-based raw material gas, and a dopantgas onto the SiC single crystal substrate to form an epitaxial filmserving as the high concentration layer, and the epitaxial film isformed under a condition that an average value of an n-type dopingconcentration of the film is 1×10¹⁸/cm³ or more and 1×10¹⁹/cm³ or less,and a condition that a C/Si ratio, which is a molar ratio of C atoms inthe C-based raw material gas to Si atoms in the Si-based raw materialgas, is 1.1 or more and 1.7 or less.
 12. The method for manufacturing aSiC epitaxial wafer according to claim 10, wherein the chemical vapordeposition device has a plurality of Si-based gas supply pipes and aplurality of C-based gas supply pipes wherein positions of the pipeswhich supply the gases to the substrate are adjustable in an in-planedirection of the substrate.
 13. The method for manufacturing a SiCepitaxial wafer according to claim 12, further comprising, before thestep of forming the epitaxial film, a step of determining a position ofthe C-based gas supply pipes of the chemical vapor deposition device,and a step of determining a position of the Si-based gas supply pipesthereof.
 14. The method for manufacturing a SiC epitaxial waferaccording to claim 13, wherein the step of determining the position ofthe C-based gas supply pipes includes: a first sub-step of supplying thedopant gas under a condition that a carrier concentration of an obtainedepitaxial film becomes less than 1×10¹⁷/cm³ and supplying the Si-basedgas and the C-based gas under a condition of a C/Si ratio of 0.6 to 0.9onto a SiC single crystal substrate using the chemical vapor depositiondevice to obtain the epitaxial film; a second sub-step of confirming afilm thickness distribution of the epitaxial film obtained in the firstsub-step; a third sub-step of determining a position of the C-based gassupply pipes without moving them in a case in which the film thicknessdistribution is 10% or less and moving the position of the C-based gassupply pipes in a case in which the film thickness distribution is not10% or less; and a fourth sub-step of repeating the first to thirdsub-steps until the film thickness distribution becomes 10% or less, andthe step of determining the position of the Si-based gas supply pipesincludes: a first sub-step of supplying the dopant gas under a conditionthat a carrier concentration of an obtained epitaxial film becomes lessthan 1×10⁷/cm³ and supplying the Si-based gas and the C-based gas undera condition of a C/Si ratio of 1.1 to 1.2 onto a SiC single crystalsubstrate using the chemical vapor deposition device, in which theposition of the C-based gas supply pipes has been determined in the stepof determining the position of the C-based gas supply pipes, to obtainthe epitaxial film; a second sub-step of confirming a film thicknessdistribution of the epitaxial film obtained in the first sub-step; athird sub-step of determining a position of the Si-based gas supplypipes without moving them in a case in which the film thicknessdistribution is 10% or less and moving the position of the Si-based gassupply pipes in a case in which the film thickness distribution is not10% or less; and a fourth sub-step of repeating the first to thirdsub-steps until the film thickness distribution becomes 10% or less.